The invention relates to devices for observing bodies, for example living bodies or manufactured articles.
It relates especially to devices for observing an eye.
Many devices are known for observing an eye, such as biomicroscopes, ophthalmoscopes or retinographic devices. They make use of various types of examinations, such as direct anatomical observations or scanning by means of dyes, such as fluorescein, indocyanin green, rose bengal or acridine orange.
With certain recent experimental apparatuses, functional scanning is also possible, such as the study of attachments or the projecting of tests intended for the recording of electrical activity of visual pathways. In this case, an image is formed directly on the retin a by means of modulation of an illumination beam.
The most conventional apparatuses are based on overall illumination with a field of 5xc2x0 to 60xc2x0 in polychromatic or monochromatic light.
In general, these apparatuses need to separate the illumination and observation pupils according to the so-called Gullstrand principle.
Retinographic devices use a photographic sensitive surface. They require much higher levels of retinal illumination than those required for ophthalmoscopic observation directly via the observer""s retina. Their level of performance is essentially defined by the optical resolution of the system.
More recently, sensitive surfaces have appeared which allow videoscopic recordings at wavelengths visible or nonvisible to the human eye. The resolution of these apparatuses is therefore subject, in addition to the optical limitations, to the limitations of the electronic acquisition.
In the field of the scanning of the cornea and the lens, illumination systems are known whose illumination source has the form of a circle or of a slit of variable width and whose optical image is focused onto the plane of observation.
In such devices, an observation system consisting of a monocular or binocular magnifying system fastened to the illumination device is used. The image of tissues may be observed either directly or via an additional lens placed close to the eye. These apparatuses meet the provisions of a pupil separation principle, called the Gullstrand principle.
More recently, corneal and/or lens microscopy apparatuses have been described. These apparatuses are very similar to those in conventional microscopy. These apparatuses use the fact that the anterior media of the eye make it possible to work with very wide beam angles, using what is called the immersion technique.
Recently, a novel technique called confocal microscopy has also been proposed for displaying the cornea. The confocal technique has the advantage of selecting a plane of optical section having a certain thickness in Z. Apparatuses making use of this technique therefore have tomographic properties, that is to say they make it possible to isolate a plane of observation in a scattering medium. Videoscopic techniques allow acquisition in real time of a large number of optical sections in successive planes.
In the field of retinal scanning, it has also been proposed to use a technique based on principles different from the above principles defined by Helmholtz in the middle of the nineteenth century.
U.S. Pat. No. 4,213,678 by Pomerantzeff and Webb has thus proposed illuminating a small area scanning the field of view. This patent teaches the use of optomechanical scanning for deflecting, in two dimensions, a light beam having a diameter of less than that of the eye""s pupil. The apparatus collects, at full pupil, the flux reflected and/or scattered by the ocular tissues. This apparatus is consistent with the Gullstrand principle, thereby limiting the resolution by using a reduced illumination pupil. This document proposes to remedy the presence of reflections by using polarized light.
In patent EP 145 563, Cohen Sabban, Roussel and Simon propose to make the flux reflected and/or scattered by the ocular tissues follow the same path as the illumination flux. This reflected flux follows the optical deflection path and thus becomes immobile. The authors call this action beam stabilization. This optical device permits filtration of the return beam coming from the eye, using spatial filtration elements in a plane conjugate with the source and with the optical tissues observed. This description corresponds to the use of a confocal device. This optical device makes it possible to eliminate reflections without using the Gullstrand pupil separation principle. 
The use of the same pupil for the illumination and observation paths gives better resolution than the device of document U.S. Pat. No. 4,213,678. The use of confocal filtering makes it possible to increase the contrast by eliminating the flux scattered by the planes superjacent and subjacent to the plane observed.
In 1987, Webb and Hughes, in patent EP 223 356, adopt the principle of stabilizing the illumination and observation beam, but they retain the separation of the pupils in the stabilized return beam. This device is more effective than the previous one with regard to the elimination of pupillary reflections, but it still has the drawback of collected flux limitation.
The use of spatial filtering devices in the stabilized return path makes it possible to obtain images whose depth of field is defined by the diameter of the filtration pupil. This tomographic aspect permits the construction of three-dimensional images of the tissues studied. However, the resolution obtained is tied to the geometrical aberrations and to the fluctuations in the transparent media. It remains limited to 30 xcexcm over the XY area of the retina and to 300 xcexcm in the case of the depth Z of the retinal tissue.
In the field of corneal and retinal scanning, Izaat et al have proposed scanning by interferometry with a device of the Michelson interferometer type.
The latter uses an illumination source of low spatio-temporal coherence. The depthwise Z resolution is determined by the coherence characteristics of the source. The use of a light-emitting diode allows a 15 xcexcm Z resolution in the eye and 20 xcexcm Z resolution at the cornea.
The image of an optical section is obtained, in Z retinal depth, by a succession of interference patterns produced between the flux coming from the tissues to be scanned and a reference flux coming from a mirror placed in what is called the reference arm. Each position of the reference mirror provides an interference system which will be scanned and will provide the information for a pixel in Z.
Scanning along a line is obtained by optomechanical scanning, making it possible to scan a new position when scanning in Z for a previous position has been completed. The XYZ resolution is of the order of 20 xcexcm under the best conditions.
This device is sensitive to the movements of the eye, since the acquisition time is relatively long compared with the bandwidths of the ocular movements. Correction algorithms are able only partially to compensate for this shortcoming. This device gives an image of various relative positions and does not make it possible to give a faithful and absolute topographic image. The actual resolution obtained is of the order of 100 xcexcm in X or Y and 50 xcexcm in Z.
In the field of adaptive optics, it has been proposed in patent U.S. Pat. No. 5,777,719 to insert a wavefront compensation device using a deformable mirror in a conventional retinographic system. One point on the surface is illuminated by a superluminescent diode which makes it possible to measure the distortions of the wavefront and to calculate the compensation which must be applied by the deformable mirror. This compensation is carried out over a complete image of the fundus of the eye, and acquired by a CCD camera. This technique makes is possible to obtain an XY resolution of 2 xcexcm, which is unsatisfactory.
In addition, this device does not make it possible to extract a useful signal corresponding to a chosen optical plane of the tissue studied by avoiding all reflections and backscattering coming from the subjacent and superjacent planes.
An overall objective of the invention is to provide a device for observing a body, such as the body of a living person or such as a manufactured article, having a particularly improved image resolution over those provided by the existing devices.
This objective is achieved according to the invention by virtue of a device for observing a body, especially an eye, comprising means for illuminating the body, means for sampling the light coming from an illuminated region of the body and compensation means capable of measuring a wavefront distortion of a beam coming from a point on the body and in applying, to the light sampled by the sampling means, a correction calculated from this measured distortion, characterized in that the sampling means and the compensation means are arranged so that, for each point on the body used for a correction calculation, the sampling means sample the light over a chosen region of the body sufficiently close to said point of calculation for the wavefront distortion of the light coming from any point in this region is the same as that coming from the point of calculation to within a fraction of a wavelength, and in that the device includes means for automatically changing the point on the body used for the correction calculation and for changing, synchronously, the associated sampling region in such a way that the device scans the observed body in the form of a series of such successive regions for each of which the distortion of the wavefront emanating from any point in the region is identical, to within a fraction of a wavelength, to the distortion of the wavefront emanating from an associated point of calculation.